GEOPHYSICAL LABORATORY. 153 



resistance-measuring system is of course in greater danger still. All such 

 trouble is absolutely prevented by an equipotential shield, which is merely a 

 connected system of metal plates, wires, etc., which interposes itself at every 

 point of solid contact between the measuring system and external bodies. 

 This shield need not be, and preferably should not be, "earthed." 



Slight modifications of this shield are also useful in electric furnaces, in the 

 measurements upon power circuits, and %vithin the potentiometer circuit itself. 



These arrangements are easy to install, most of them require no subsequent 

 attention, and all are easily tested. 



(36) Thermo-elements of precision, especially for calorimetry. Walter P. White. J. Am. 



Chem. Soc, 36, 2292-2313 (1914). 



Inhomogeneity, once a serious foe to precision in thermo-elements, and still 

 often supposed to be such, can without difficulty be rendered practically 

 negligible in copper-constantan thermo-elements used for any precision up to 

 50 parts per million. Such thermo-elements, accordingly, may, except for 

 imperfect insulation, easily preventable, be free from all appreciable errors 

 other than those (such as incomplete depth of immersion) which are possible 

 Anth all thermometers. To attain this freedom from error the wire used must 

 be tested, and the essential though easily satisfied requirements peculiar to a 

 thermo-electric system must be observed. These requirements this paper 

 attempts to consider in detail, and it also describes simple but important 

 details regarding the operations of construction, insulation, inclosure, calibra- 

 tion, etc., of the thermo-elements. 



Constantan wire for thermo-elements has been so far improved that con- 

 tinuous lengths are frequently obtainable which vary (in electro-motive force 

 against copper) less than 0.0002, making sensitive thermo-elements with 

 errors usually less than 20 per million. 



The testing of wire enough for a thermo-element of maximum sensitiveness 

 takes but an hour or two, 'with simple apparatus. 



On account of the ease with which thermo-elements can be constructed, the 

 more sensitive combination of several couples is generally preferable to a 

 single couple, even for cruder measurements. 



(37) Easy calorimetric methods of high precision. Walter P. White. J. Am. Chem. Soc, 



36, 2313-2333 (1914). 



In the calorimetric method of mixtures, a precision approaching or reaching 

 0.1 per mille, though somewhat unusual, is often desirable, and is ordinarily not 

 difficult to attain with appropriate apparatus. Its attainment is especially 

 easy with a two-calorimeter installation, which secures the convenience and 

 high precision of differential thermo-electric temperature measurement. This 

 is the only advantage of the two-calorimeter arrangement; the diminution of 

 heat-loss error, often counted an advantage, turns out upon examination to be 

 largely illusory. By abandoning the twin calorimeters previously used to get 

 this supposed advantage, and using for the comparison calorimeter a vacuum- 

 jacketed ffask, there is a gain in convenience and precision. A special thermo- 

 element combination renders the necessary temperature observations as simple 

 as Avith the twin arrangement. A completely inclosing jacket of uniform 

 temperature is necessary for this method, but this is no loss, for such a jacket 

 proves to be necessary for highest precision with any other method. This 

 method is quite as effective with two jackets, one around each calorimeter, 

 and therefore with adiabatic methods. 



Efficient complete jackets can be very easily realized according to several 

 methods, which are described. 



